I know the following without evidence is going to seem well just not true
I have a friend who was a respected expert , almost master ,who recently left the game for other pursuits (not because of f4 though partly because he felt he had to tackle another opening). Through computer analysis (and his own refutation attempts at the refutation) he thought he discovered a forced draw in evry variation (or a better position ofr black) after a e5 gambit line of black (my apoligies i cant remember the name). I didnt come up with it but he showed us at the coffee house countless times (with his laptop sometimes) different amazing tactical resources for black , sacrficing exchanges, pawns, pieces etc...
Anyway he was convinced that grandmasters new about these lines (maybe thats why they dont play it) and dropped f4 because of his disappointing findings for the opening he loved
1. f4 is alive and well. Still played by grandmasters with plenty of wins and losses.
I agree the opening does have its merits and probably is played occasionally by GMs.
It is also is true that the opening is not played by players 2700+ much or at all except maybe in simuls or 5 min or bullet chess.